One of the most theoretically effective methods for effecting dehydration of low rank coal, such as lignite and sub-bituminous coal, is steam drying. Low rank coals can typically have between about 30-65 percent water bound in the coal. This water reduces the BTU value per unit of weight of the coal, increases transportation costs, and makes handling more difficult. Therefore a method of efficiently dehydrating the coal is highly desirable.
Conventional processes for steam drying of low rank coal have not met with great commercial success. Typical prior steam drying processes are discussed in detail in report DOE/GFETC/RI-82/1 from the Technical Information Center of the U.S. Department of Energy entitled "Steam Drying of Lignite Coal and a Review of Processes and Performance" by Stanmore et al. This report points out that while batch processes for steam drying coal have been used commercially in the past, there has not been a commercially successful continuous process for steam drying of coal.
According to the method and apparatus of the present invention, it is possible to effectively continuously dehydrate coal utilizing a steam/water phase dehydrator. A key to the ability of the invention to accomplish this result is the ability to introduce and remove coarse particles of low rank coal into a pressure vessel (the steam/water phase dehydrator) continuously. The process and apparatus according to the present invention are advantageous since not only do they effect removal of about 60-80 percent of the water in low rank coal, they do so utilizing mostly commercially available equipment, efficiently utilize heat at all stages so that there is a minimum of energy use, and remove some sulfur, sodium, and other mineral contaminants from the coal, which contaminants can foul boilers and the atmosphere when the coal is burned during ultimate use thereof. Despite the activity in the field of low rank coal dehydration, and the pressing need for a commercially feasible continuous coal dehydration process, prior to the invention it had not been recognized that equipment commercially available for many years in the pulp and paper art could be adapted to effect continuous coal dehydration.
According to the present invention, low rank coal is particlized to a particle size of less than about 4 inches, and preferably less than about 2 inches. In the most specific procedures according to the invention, the particlized coal is steamed atmospherically, steamed again in a preheating vessel at a pressure of between about 20-100 psig, and entrained in water in a chute leading to a high pressure feeder. The high pressure feeder transfers the coal/liquid slurry to a particles/liquid separating structure at the top of a vertical pressurized dehydrator, the pressure in the dehydrator being maintained between about 300-1500 psig. Saturated steam contacts the coal particles at the top of the dehydrator, and condensed steam and coal particles move downwardly in the dehydrator in a column.
Wash water is introduced at the bottom of the dehydrator to effect washing and cooling of the coal particles at the bottom of the dehydrator. The spent wash water is flashed into steam, which is used for the previous atmospheric steaming steps, and at other stages in the process, and sulfur, sodium, and like minerals are ultimately removed from the spent wash water. The coal particles are discharged from the bottom of the dehydrator to the high pressure loop of a second high pressure feeder, which transfers the particles to the low pressure loop thereof and ultimately to a particles/liquid separator which operates at a pressure of between about 20-100 psig. The particles from the liquid/particles separator are subsequently passed to a vibrating dryer or the like, where they are subjected to evaporative cooling. Moist discharge gases from the vibrating dryer pass through a gas cyclone or the like to remove coal fines, which are combined with the coal particles discharged from the vibrating dryer to provide a dry, dehydrated, low rank coal suitable for easy transportation and efficient burning in conventional boilers and the like.
It is the primary object of the present invention to provide an effective continuous method--and apparatus for practicing the method--for dehydrating low rank coal, such as lignite and sub-bituminous coal. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.